Barry,
Yes, most of the guy wires are just to anchor the tree. No advantage other than it's quick any easy.
Well.. it also makes it easy to adjust the planting angle. I put a layer of soil on the bottom of the box, place the tree and wire it down. I get the tree tight where I want it and fill in the rest of the way.
I guess another thing would be, by tying the tree up higher it's held very firmly when I use guy wires to pull branches and manipulate branches ( a couple of wires on this tree are pulling branches).
I only do this in training/grow boxes....After a few months the tree anchors itself in the box and the wires come off.
In a bonsai pot I also would secure the tree by wiring the roots in the pot.
Jay